Hydraulic systems

ABSTRACT

A hydraulic system including a compensator connected between the service ports of the control valve and the load and arranged to restrict the flow in one or both of the service lines so as to maintain a constant pressure difference between the supply pressure and the pressure at the valve service port supplying fluid to the load. The compensator may be connected in both service lines and arranged to restrict the flow equally in each of them. Alternatively a single-sided compensator may be used, and if the load is liable to over-run it must be connected in the service line which is downstream of the load when over-running.

This invention relates to hydraulic systems in which a four-way controlvalve controls the supply of hydraulic power to move a load.

In such systems, the response to movements of the control valve dependson the magnitude of the load, and where the load may vary within widelimits the system may be difficult to control precisely and reliably. Anobject of the present invention is to reduce this difficulty.

In some cases the load may tend to overrun, causing cavitation in thesupply lines, and the preferred form of this invention provides a meansof preventing cavitation and enabling smooth control to be maintainedwith an over-running load.

In a system according to the present invention, a compensator isconnected between the service ports of the control valve and the load,and is arranged to restrict the flow in one or both of the service linesso as to maintain a constant pressure difference between the supplypressure and the pressure at the valve service port supplying fluid tothe load. The compensator may be connected in both service lines andarranged to restrict the flow equally in each of them. Alternatively asingle-sided compensator may be used, and if the load is liable toover-run it must be connected in the service line which is downstream ofthe load when overrunning.

In a preferred form of the invention the control valve is apilot-operated servo valve and the pilot pressure is applied to operatea shuttle valve so as to select the service line supplying fluid to theload and to apply the pressure in this line to the compensator. Wherethis is not convenient, pilot pressure for operating the shuttle valvemay be obtained from a flow-sensing valve in one of the service lines.

In a preferred form of construction the compensator and the pilotoperated shuttle valve are incorporated in a block provided with portson its opposed flat surfaces corresponding to the ports of the controlvalve so that it can be assembled between the control valve and asurface having the supply and load connections without modification ofthe control valve.

When a number of systems according to the invention are connectedtogether in an installation provided with a common power supply, shuttlevalves may be connected across the load of each system to provide a loadsensing supply for control of the supply pressure, for example, bycontrolling the off-load point of variable off-loading valve or thepressure compensator setting and hence the swash angle of a variabledelivery swash plate pump.

The invention will be further described by way of example with referenceto the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a system according to the invention.

FIG. 2 shows a modification of the circuit of FIG. 1.

FIG. 3 is a plan view of a unit incorporating a compensator and shuttlevalve connected in a circuit according to the invention.

FIG. 4 is an elevation of the unit of FIG. 3.

FIGS. 5, 6, 7, 8, 9, and 10 are sections of the unit taken on the planesindicated in FIG. 4.

Referring first to FIG. 1, a control valve 1 controls the supply ofpower to a load 2. In the FIGURE this load is shown as a ram, althoughit could, of course, be any other suitable form of actuator, for examplea vane motor. The valve 1 is a four-way valve having the inlet 3connected to the supply line, a pair of tank ports 4 and 5 exhausting totank, and a pair of service ports 6 and 7 which by movement of the valvespool 8 are connected, one of them to the tank, and the other to thesupply. The valve 1 is a pilot operated servo valve, the pilot stage ofwhich is not shown in the drawing. A separate pilot valve may be usedbut in a convenient construction the pilot valve is contained within themain valve spool 8. In either case, the pilot pressure for operating themain valve spool appears across the end faces of the spool 8 at 9 and10.

The supply from the service ports 6 and 7 is applied to ports 11, 12 ofthe load and according to the invention this is done by way of acompensator 13.

The compensator 13 is a spool valve having a pair of ports 14, 15connected to the service ports 6, 7 of the servo valve and a pair ofoutlet ports 16, 17 connected to the ports 11, 12 of the load. The spool18 of the compensator has a pair of raised lands 19, 20 arranged toco-operate with the ports 14, 15 so as to control the supply ofhydraulic fluid into and out of the service ports 6 and 7 of the controlvalve. One end 21 of the compensator spool 18 is acted on by the supplypressure, the other end of the spool 18 is subjected to the pressure inone of the valve service lines from a shuttle valve 22, and also to theforce of a biasing spring 23.

The shuttle valve 22 is a spool valve in which the ends of the spool areacted on by the pilot pressures operating at the ends 9 and 10, of thecontrol valve, the spool moving under this pressure so as to connect theend space of the compensator to whichever of the service ports 6 and 7is being connected by the control valve to the supply line.

In operation the spring 23 is set up so as to correspond to the pressureto be dropped across each opening of the control valve 1, this pressurebeing fairly low, for example 125 lbs. per sq. in. When the controlvalve is moved, for example to the left, in the view shown in theFIGURE, pilot pressure in the valve also operates the shuttle valve 22connecting the service port 6 to the end space of the compensator 13where it acts on the compensator spool 18 in addition to the force ofthe spring 23 so as to oppose the supply pressure applied to the otherend 21 of the spool. When the load starts to move and if the pressure atthe service port 6 falls, the supply pressure overcomes these combinedpressures, moving the spool 18 to the left and causing the lands 19, 20to close the ports 14, 15 so restricting the the flow of hydraulic fluidas to maintain pressure in the service port 6 at the predetermined dropof 125 lbs. per sq. in. below line pressure set by the spring 23. Therate of movement of the load is therefore set by the opening of acontrol valve 1 independently of the magnitude of the load at anymoment.

Alternatively, if due to an opposing load the pressure at the serviceport 6 rises, the compensator spool moves to the right to maintain thepressure drop in the service line at 125 lbs. per sq. inch.

Should the load tend to over-run, the pressure in the service port 6drops, causing the compensator to close, restricting the flow ofhydraulic fluid in both service lines so as to maintain this pressuredifferential so that no cavitation occurs and the load moves undercontrol of the valve 1.

If a number of systems of this kind are to be driven from a commonsupply whose pressure is controlled by load-sensing means, a ballshuttle valve 24 is connected across the load ports 11 and 12 and thepressure from this is carried through line 25 to the power unit ofanother system where it may be used to control the unloading point of anoff-loading valve or the pressure compensator setting and therefore theswash angle of a variable delivery swash plate pump or other similarcontrol device.

The arrangement described above with reference to FIG. 1 uses the pilotpressure of the main servo control valve for operating the shuttlevalve. This may sometimes not be convenient; for example the maincontrol valve may not be pilot operated. In such a case the inventionmay be carried out with the modification which will now be describedwith reference to FIG. 2.

Referring now to this FIGURE, in one of the service lines to the load 2a flow polarity sensing valve 30 is inserted to provide the necessarypilot pressures to the shuttle valve 22. The polarity sensing valve 30comprises a ball 31 which is a free fit in an aperture in a plate 32 andwhich is maintained central in this aperture by means of a pair ofsprings 33, 34. The pilot pressures for operating the shuttle valve 22are taken from either side of the plate 32. In operation, any flow intoor out of the port 12 of the load 2 has to pass through this valve anddoes so by displacing the ball 31 to one or other side of the positionin which it closes the aperture in the plate 32 against the action ofone of the springs 33, 34. A pressure drop is therefore generated acrossthe valve 30 whose magnitude depends primarily on the stiffness of thesesprings, and this pressure drop is applied to the ends of the spool ofthe shuttle valve 22 which is arranged to transmit the pressure of thevalve service line to the compensator 13 of FIG. 1.

In a common construction of a hydraulic control system the control valveis mounted directly on the power unit. In this case both the controlvalve and the power unit have matching surfaces machined flat in whichare formed galleries for carrying the supply service and pilot controlpressures between the valve and the power unit. These end at eachmatching surface in a matching system of galleries and either eachopening in one of the surfaces is surrounded by an O-ring seal so thatwhen the valve is bolted to the power unit the necessary connections areautomatically made, or a separate sealing plate is provided.

In a preferred manner of carrying out the present invention thecompensator and shuttle valve, and load-sensing shuttle if required, aremounted in a unit comprising a rectangular block having machined flatopposed surfaces provided with a corresponding system of passages sothat it may be bolted directly between the control valve and the powerunit to form a system according to the present invention. The block thuscontains the portion within the dotted lines of FIG. 1.

FIG. 3 shows a unit of this kind in plan, and FIG. 4 shows the same unitin side elevation.

Referring first to FIG. 3, the unit comprises a rectangular block 40 ofwhich the top face (visible in the FIGURE) and the parallel bottom faceare machined flat and provided with corresponding openings. A projectionfrom the block 41 forms a housing for the compensator spring of FIG. 1.The openings visible in the FIGURE comprise a set of four holes, 42, 43,44, 45, which extend right through the unit and accommodate tie bolts bymeans of which the valve and the compensator unit are firmly secured tothe power unit. A passageway 46 extends right through the unit andprovides a connection for system pressure from the pump and a similarparallel passage 47 provides the connection to tank for the servo valveand compensator. Two smaller passages 48 and 49 allow pilot pressure tobe transmitted from the servo control valve to the shuttle valve whichis within the unit 40.

The openings 50 and 51 form part of the service line. These openings donot extend right through the unit but are drilled in for a shortdistance from each face. Cross drillings from the side of the unitprovide the necessary connections on the valve side and the power unitside to the ports of the compensator. A locating hole 52 may be providedto assist in location during assembly.

FIG. 4 is a side elevation of the block 40 showing a number of drillingswhich form internal passageways for the interconnection to thecompensator and shuttle valve located within the block. The outer endsof each of these drillings is sealed by a screwed plug.

Referring now to this FIGURE, the passageways 55 and 56 provide inletsfrom the servo valve service port to the compensator and 57 and 58provide outlets from the compensator back through connections on thepower unit to the load; 59 and 60 allow the pilot pressures from thepassages 48 and 49 of FIG. 3 to the ends of the shuttle valve which iswithin the block 40. The drilling 63 is part of a system of connectingpassages which apply system pressure to the end of the compensator spoolremote from the spring. A locating pin 78 is provided to assist locationduring assembly.

FIGS. 5 and 6 are cross-sections of the block 40 in the plane 5--5 and6--6 respectively indicated in the FIG. 4. Referring now to thoseFIGURES, the compensator valve comprises a spool working within a bore65. Similarly the shuttle valve comprises a spool working within afurther bore 66.

The service line connection from the control valve 50 is only drilled ashort distance into the block, where it meets the cross drilling 55.Further drillings not shown in FIG. 5 connect this to the appropriateposition in the compensator bore so that it forms an inlet port for thecompensator valve. The compensator outlet port is formed by the drilling57 which communicates with the service line outlet 67. Drillings 55 anddrilling 56 shown in FIG. 7 connect the service ports to the ports inthe shuttle valve bore. The pressure line 46 and the tank line 47 aredrilled straight through the block 40. A connection exists between thepressure line and one end of the compensator, but this is not visible inthe section of FIG. 5. A drilling 53 passes through the shuttle valvebore 66 to make connection with a passageway 69 conveying the pressureselected by the shuttle valve to the spring end of the compensator.

FIGS. 7, 8, 9, and 10 are sections of the unit on the planes indicatedin FIG. 4. These sections show the compensator and shuttle valve and thevarious internal passages by which they are interconnected.

Referring first to FIG. 7, the service line connections from the valve50, 51 are connected by passageways 55 and 56, and the cross drilling 70to passages 72 and 73 which provide the inlet ports of the compensatorvalve. This valve will be shown more fully in FIG. 9.

FIG. 8 is a section taken in the plane of two of the passageways 76, 77by means of which the supply pressure is transmitted to the end of thecompensator spool opposite to the spring.

The section of FIG. 9 is taken through the axis of the compensator spool18. One end of the spool is acted on by the bias spring 23 contained inthe housing 41. The other end of the spool is subject to system pressurefrom the pressure line 46 by way of the passages 76 and 77 shown in FIG.8. The service lines 67, 68 which supply the load communicate with thepassages 57, 58 which pass through the valve bore 65 to form the outletports of the compensator. The spring housing 41 is also connected to theload pressure from the shuttle valve by means of a system of passages, apart of which 53, 69 appears in FIG. 5.

FIG. 10 is a section taken in a plane which includes the axis of theshuttle valve.

The shuttle valve spool 22 works in the bore 66 and is acted on ateither end by pilot pressures from the pilot pressure lines 48 and 49which are conveyed to the ends of the shuttle valve spool by means ofthe passages 59 and 60. The service lines from the control valvecommunicate with the passages 55 and 56 (not shown in FIG. 10) whichbreak through the bore 66 and so provide ports for the selection of therequired service line pressure as determined by the pilot pressures. Theoutput from the shuttle valve is applied through the passages 53 and 69,(see also FIG. 5) to the spring end of the compensator.

The above arrangement may be simplified by using a compensator with asingle land metering the flow in only one of the service lines. If theload is liable to overrun the compensator should be in the line which isdownstream of the load in the over-running condition.

The unit as described above does not include any connection for aload-sensing line. If this is required it may be provided by theinclusion of a ball shuttle valve connected to the lines 67, 68 so as torespond to the higher of the pressures in the two lines.

Seals will of course be required at the surfaces of the unit around eachof the bores to prevent the escape of fluid, and these have been omittedfor clarity.

I claim: .[.1. A hydraulic system including a source of hydraulicpressure, a load, a four-way control valve arranged to control thesupply of hydraulic pressure to said load, said valve having a pair ofservice ports, a pair of service lines connecting said service ports tosaid load, a compensator being connected in at least one of said servicelines and being arranged to restrict the flow of hydraulic fluid in saidservice line so as to maintain a constant pressure difference betweensaid source pressure and said pressure at that one of the valve serviceports that supplies fluid to said load..].
 2. A hydraulic system.[.according to claim 1 in which the compensator is connected in bothservice lines and is arranged to restrict flow equally in each ofthem.]. .Iadd.including a source of hydraulic pressure, a load, afour-way control valve arranged to control the supply of hydraulicpressure to said load, said valve having a pair of service ports, a pairof service lines connecting said service ports to said load, acompensator being connected in at least one of said service lines andbeing arranged to restrict the flow of hydraulic fluid in said serviceline so as to maintain a constant pressure difference between saidsource pressure and said pressure at that one of the valve service portsthat supplies fluid to said load,said compensator being connected inboth service lines and is arranged to restrict flow equally in each ofthem. .Iaddend.
 3. A system according to claim 2 in which the controlvalve is a pilot-operated servo valve, and the pilot pressure isarranged to operate a shuttle valve so as to select the service linesupplying fluid to the load, and to apply the pressure in that line tothe compensator.
 4. A system according to claim .[.1.]. .Iadd.2.Iaddend.including a shuttle valve, a flow-sensing valve is included inone of the service lines to provide pilot pressures for operating saidshuttle valve which selects the service line supplying fluid to theload, and to supply the pressure in this line to said compensator.
 5. Acompensator and pilot operated shuttle valve incorporated in a blockprovided with ports on opposed flat surfaces, constructed and arrangedso that it can be assembled with a control valve to form a systemaccording to claim .[.1.]. .Iadd.2.Iaddend..